An integrating sphere is used to measure the intensity of light emitted from a sample. The inner wall of the integrating sphere is made out of a coating or a material having a high reflectance and excellent in diffuseness, and light incident on the inner wall surface is multi-diffusely-reflected. Then, the diffused light from the sample enters a photodetector through an exit aperture provided at a predetermined position of the integrating sphere to be detected, and this method can obtain information of light emitting intensity and the like of the sample with high accuracy without depending on a light emission pattern, a light emission angle characteristic, and the like of the sample (for example, refer to Patent Documents 1 to 3).
An organic EL (electroluminescence) element serves as an example of the sample of a target of measurement using the integrating sphere. The organic EL element is a light emitting element generally having a structure with an anode, an organic layer including a light emitting layer, and a cathode laminated on a substrate made out of glass or a transparent resin material. Photons are generated by holes injected from the anode and electrons injected from the cathode being recombined in the light emitting layer, and the light emitting layer emits light.
In measurement and evaluation of light emission characteristics of the organic EL element, an external quantum efficiency defined by a ratio of the number of photons emitted to the outside of the element to the number of injected electrons, and the like, becomes important. Further, in measurement and evaluation of a luminescent material used in the organic EL element, a luminescence quantum yield (internal quantum efficiency) defined by a ratio of the number of photons of light emitted from the sample to the number of photons of excitation light absorbed by the sample becomes important. A light measurement device using the integrating sphere can be preferably used for evaluation of quantum efficiency of such an organic EL element.